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The Bornholm region

Theme: Block tectonism  

Locality: The Bornholm region.
Description of a Danish GeoSite

Primary identification

1. GeoSite no.:
GS - 9-1.

2. National locality no.:
NGI - 183-197.

3. GeoSite name:
Bornholm and surrounding marine areas.

4. Location:
Central Baltic Sea.

5. Co-ordinates UTM:
System ED50 Zone 32,
x=875516, y=6123113; x=883588, y=6111501;
x=891162, y=6113604; x=891051, y=6114821;
x=890885, y=6116921; x=892046, y=6120238;
x=892046, y=6123832; x=890609, y=6127868;
x=883311, y=6116479; x=885412, y=6118580;
x=886462, y=6118580; x=887955, y=6118746;
x=889116, y=6122726; x=878004, y=6117806;
x=876069, y=6116645; x=875792, y=6115263;
x=875074, y=6114599; x=873028, y=6115152;
x=870485, y=6115982; x=868771, y=6115982;
x=867721, y=6116645; x=866615, y=6117087;
x=866062, y=6117640; x=863962, y=6119299;
x=867334, y=6119409; x=867168, y=6118801;
x=865233, y=6121842; x=865731, y=6122284;
x=871646, y=6123777; x=878059, y=6131516;
x=866007, y=6132456; x=868274, y=6136437;
x=868329, y=6137377; x=868053, y=6138925;
x=864680, y=6137764; x=867279, y=6140970;
x=865731, y=6141799.

6. Type of locality:
Island with its marine surroundings.

Primary geological features

7. Main topic:
Block tectonism.

8. Geo(morpho)logy:
Block faulting in the Tornquist Zone that separates the East European Platform (craton) from the NW European Craton. The Bornholm Block and the surrounding fault block mosaic are situated in the overlap zone between Teisseyre-Tornquist Zone to the southeast and the Sorgenfrei-Tornquist Zone to the northwest.

The island of Bornholm is a composite fault block, the Bornholm Block. The bedrock on the island is composed of Precambrian crystalline basement outcropping to the north; and a complex mosaic of fault blocks outlined by Lower Palaeozoic platform sediments to the south, while Mesozoic sediments are mainly encountered to the west. The overall dip of the Palaeozoic platform deposits is to the southeast, whereas the Mesozoic strata predominantly dip towards southwest. The "Palaeozoic" fault blocks are bounded by WNW-ESE and NNW-SSE trending faults. The bounding faults of the "Mesozoic" blocks follow the same trends in part, but the fault orientations have a wider scatter and an additional NW-SE trending segment. On southern Bornholm the Mesozoic is separated from Palaeozoic fault blocks by an angular unconformity. Together the divergent dip directions, the different fault orientations encountered in the "Palaeozoic" and the "Mesozoic" fault blocks, and the discordant Mesozoic/Palaeozoic relationship suggest that faulting of the Bornholm area took place in different tectonic regimes; early faulting during the Late Palaeozoic and renewed faulting during the Mesozoic. The Late Palaeozoic faulting presumably took place in the Late Carboniferous - Early Permian period when widespread faulting and associated volcanic activity in the Variscan foreland took place. In the Fennoscandian Border Zone (see below), episodes of extension and volcanic activity are expressed by the swarm of intruded basic dykes.
The Bornholm Block is an integral part of a larger scale horst and graben zone, and it is surrounded by the Christiansø Block to the north, the Rønne Graben to west. To the South it is flanked by the Arnager Block, the Risebæk Graben, and the Darlowo Block. The fault zone with horsts and grabens vanishes to the southeast, while it extends to the northwest across Skåne (Scania) in Sweden as part of the Fennoscandian Border Zone.
On a regional scale Bornholm and the Fennoscandian Border Zone are included in the Tornquist Zone. The NW-SE trending Tornquist Zone is a deep-seated fault zone that separates the East European Craton (composed of the Baltic Shield and the East European Platform) to the northeast from the NW-European Craton to the southwest. The Tornquist Zone is divided into the Teisseyre-Tornquist Zone to the southeast and the Sorgenfrei-Tornquist Zone the northwest. The Fennoscandian Border Zone outlines the southeast segment of the Sorgenfrei-Tornquist Zone. The Bornholm Block is situated in the centre of the Tornquist Zone in the overlap area between its southeastern and northwestern segments. The dominant NW-SE trend of the Tornquist Zone is broken in the overlap zone by the N-S trending Rønne Graben, which connects the two primary segments.
In Late Cretaceous time the entire Tornquist Zone underwent structural inversion followed by erosion during the Cainozoic.

9. Frame / context:
Structural Geology.

10. Chronostratigraphy:
Main tectonic phases occurred during the Palaeozoic and Mesozoic.

11. Primary value:
In the Bornholm area, the structural inversion of the Rønne Graben, associated with fault block tectonics, displays some of the most complete inversion sections known in marine reflection seismic profiles. In contrast to the remaining Tornquist Zone, the Cenozoic erosion in the southern Rønne Graben is limited.

12. Comparative evaluation:
The Tornquist Zone is a unique and prominent crustal scale, intra-continental NW-SE trending tectonic lineament that separates the East European Craton / Baltic Shield from the NW-European craton. The fault zone is c. 2.000 km long and extends from Skagerrak in the eastern North Sea Basin to the northwest to the Black Sea in the southeast. The central horst - graben system in the Bornholm area, and the continuation across Scania (Sweden) into Kattegat to the northwest, outline a c. 220 km long zone of horst blocks. The fault zone is easily accessible, and a comparable tectonic landscape is not known from other parts of Europe.

13. Comparable GeoSites:
In Denmark: None
Outside Denmark: Horst blocks in Scania.

Supplementary data in support of the locality

14. Delineation on a map:

15. Higth of the GeoSite:
Up to c. 147 metres above sea-level and the surrounding landscape.

16. Area of the GeoSite:
Bornholm region proper is c. 2000 km2.

17. Subordinate geological interests:
Documentation of the stratigraphic record of the Danish subsurface. Many plutonic and sedimentologic specialities. And as for the National Geological locality no. 194 (Læså) and GeoSite no. 8-1 (Western Bornholm).

18. "Non"-geo interests:
Interests of the rural society.

19. References:
Erlström, M., Thomas, S.A., Deeks, N. & Sivhed, U. 1997: Structure and tectonic evolution of the Tornquist Zone and adjacent sedimentary basins in Scania and the southern Baltic Sea area. Tectonophysics, 271, 191-215.
EUGENO-S Working Group 1987: Crustal structure and tectonic evolution of the transition between the Baltic Shield and the North German Caledonides (the EUGENO-S Project). Tectonophysics, 150, 253-348.
Graversen, O. 2004a: Upper Triassic-Lower Cretaceous seismic sequence stratigraphy and basin tectonics at Bornholm, Denmark, Tornquist Zone, NW Europe. Marine and Petroleum Geology, 21, 579-612.
Graversen, O. 2004b: Upper Triassic-Cretaceous stratigraphy and structural inversion offshore SW Bornholm, Tornquist Zone, Denmark. Bulletin of the Geological Society of Denmark, 51, 111-136.
Gravesen, P., Rolle, F., & Surlyk, F. 1982: Lithostratigraphy and sedimentary evolution of the Triassic, Jurassic and lower Cretaceous of Bornholm, Denmark. Geological Survey of Denmark, Series B, 7, 51 pp.
Gry, H. 1969: Megaspores from the Jurassic of the island of Bornholm, Denmark. Bulletin of the Geological Society of Denmark, 19, 69-89.
Hamann, N.E. 1987: Bornholm, geological map of Mesozoic formations. National Forest and Nature Agency, Marine Raw Material Survey, Denmark, 1 map.
Jensen, J.B. 1988: Bornholm sydvest - en detailundersøgelse. Havbundsundersøgelser, geologi, Skov- og Naturstyrelsen, 51 pp.
Jensen, J.B., & Hamann, N.E. 1989: Geological mapping of Mesozoic deposits along th eastern margin of the Rønne Graben, offshore Bornholm, Denmark. Bulletin of the Geological Society of Denmark, 37, 237-260.
Kramarska, R., Krzywiec, P. & Dadlez, R. 1999: Geological map of the Baltic Sea bottom 1:500.000. Panstwowy Instytut Geologiczny, Gdansk-Warszawa.
Liboriussen, J., Ashton, P. & Thygesen, T., 1987: The tectonic evolution of the Fennoscandian Border Zone in Denmark. Tectonophysics, 137, 21-29.
Michelsen, O., Nielsen, L.H., Johannessen, P.N., Andsbjerg, J. & Surlyk, F. 2003: Jurassic lithostratigraphy and stratigraphic development onshore and offshore Denmark. In: Surlyk, F. & Ineson, J.R. (eds): The Jurassic of Denmark and Greenland. Geology of Denmark Survey Bulletin, 1, 147-216.
Norling, E., & Bergström, J. 1987: Mesozoic and Cenozoic tectonic evolution of Scania, southern Sweden. Tectonophysics, 137, 7-19.
Petersen, H.I., Bojesen-Koefoed, J.A. & Nytoft, H.P. 1996: Depositional environment and burial history of a Lower Cretaceous carbonaceous claystone, Bornholm, Denmark. Bulletin of the Geological Society of Denmark, 43, 133-142.
Petersen, H.I., Nielsen, L.H., Bidstrup, T. & Thomsen, E. 2003: Burial depth and post-Early Cretaceous uplift of Lower-Middle Jurasic strata in the Fennoscandian Border Zone based on organic maturity. Geological Survey of Denmark and Greenland Bulletin, 1, 611-630.
Thomas, S.A. & Deeks, N.R. 1994: Seismic evidence for inversion tectonics in the strike-slip regime of the Tornquist zone, Southern Blatic Sea. Zeitschrift für Geoogische Wissenschaften, 22, 33-45.
Vejbæk, O.V. 1985: Seismic stratigraphy and tectonics of sedimentary basins around Bornholm southern Baltic. Geological Survey of Denmark, Series A, 8, 30 pp.
Vejbæk, O.V. 1997: Dybe strukturer i danske sedimentære bassiner. Geologisk Tidsskrift, hæfte 4, 1-31.
Vejbæk, O.V. & Andersen, C., 2002: Post mid-Cretaceous inversion tectonics in the Danish Central Graben - regionally synchronous tectonic events? Bulletin of the Geological Society of Denmark, 49, 129-144.
Vejbæk, O.V. & Britze, P. (eds) 1994: Top pre-Zechstein (twoway traveltime and depth), Geological map of Denmark 1:750000. Geological Survey of Denmark, Map Series, 45, 9 pp.
Vejbæk, O.V., Stouge, S., & Poulsen, K.D. 1994: Palaeozoic tectonic and sedimentary evolution and hydrocarbon prospectivity in the Bornholm area. Geological Survey of Denmark, Series A, 34, 23 pp.
Ziegler, P.A. 1990: Geological atlas of Western and Central Europe (2nd ed.): Shell Internationale Petroleum Maatschappij B.V.: Geological Society of London. Amsterdam: Elsevier, 239 pp., 56 encl.

20. Data sources, collections:
Geological Museum Copenhagen, The Nature Center of Bornholm, The Museum of Bornholm in Rønne.

21. Illustrations:
Photo: Air photo of Hammer Knuden.
Maps and X-sections to be included below

22. Proposers:
Dansk Geologisk Forening & Det Kongelige Danske Geografiske Selskab/S.A. Schack Pedersen & P.E. Nielsen.

23. Description by:
O. Graversen.

24. Recommandation:
The Bornholm region is a GeoSite.

The Mesozoic of Western Bornholm (GS - 8-1) is suggested as a GeoSite as well.

26. Supplementary description:

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Last modified : Onsdag 20. Dec., 2006
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